GB986017A - Improvements in or relating to indexing circuit arrangements for colour television receivers - Google Patents

Abstract

986,017. Colour television receivers. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. Aug. 15, 1962 [Aug. 18, 1961], No. 31312/62. Heading H4F. The invention relates to a circuit arrangement for providing a signal of suitable phase and frequency for application to the control electrode 3 of the single-beam, indexing, tube cathode-ray tube 1 whose screen 2 is provided with run-in indexing strips and with main indexing strips which in the particular embodiment shown occur after every two colour strips. The photomultiplier 4 is sensitive only to ultra-violet light and responds only when the beam scans the run-in strips and indexing strips which are made emissive of ultra-violet light. The former gives rise to a signal of frequency fh which is passed by the tuned amplifier 7 and is used to provide a phase reference signal for a frequency dividing circuit 9. The indexing strips give rise to a signal of frequency fi which is passed by the tuned amplifier 8 and divided by a factor # ( = 3) in the frequency dividing circuit 9. This signal of frequency fi/# is frequency added in the circuit 10 to a signal of frequency fr derived from the transmitted composite video signal and comprising a reference frequency signal locked on to the sub-carrier burst (fr = 4À5 Mc/s., fi=12 Mc/s., therefore #i/# +fr= 12/3+4.5=8À5 Mc/s.). In order to reduce errors caused by the second harmonic component of either of the signals fi/# or fr, the stage 10 comprises a pair of multi-grid valves connected in push-pull (Fig. 2, not shown) or an electron beam deflection tube (Fig. 3, not shown) or a four diode ring modulator. The stage 11 subtracts the signal of frequency #i/# + #r from the signal #i to provide a signal of frequency (1 - 1/#) #i -#r. This signal is frequency added in stage 13 to a signal fr + chr corresponding to the suppressed carrier chrominance signal. If desired, the chrominance signal may first be converted to dot-sequential form. Second harmonics may be eliminated in the circuit 13 by using circuit arrangements similar to those used in the stage 10. The output signal from the stage 13 is added in the stage 26 to either the monochrome signal M or the unconverted luminance signal Y. In addition to eliminating frequency errors primarily due to second harmonic components, the Specification also discloses how phase errors may be reduced by ensuring that the time constants inherent in the particular stages of the circuitory have preselected values. The final signal applied to the control electrode 3 of the tube 1 has a frequency #s which is equal to (1-1/#) #i which in itself is equal to #i/α where α is the ratio of the number of indexing strips to the number of groups of colour strips, i.e. 2/3 is the particular embodiment described.